Along with a remarkable development of the electronics technology, the computer performance has greatly advanced. For example, to perform color image processing, a large amount of data must be processed within a short period of time, which has been difficult for a conventional computer in terms of the processing speed. However, recent improvement of the computer performance makes such color image processing popular.
A color printing apparatus (to be referred to as a printing apparatus hereinafter) for outputting a color image rapidly becomes utilized over a wide range. For example, an output of a color image using a printing apparatus such as an inkjet printer is replacing conventional photo-printing. The image size widely ranges from a small namecard size to a large B0 poster size or more.
With the spread of such printing apparatuses, demands have arisen for higher image quality and higher throughput of the apparatuses. The printing apparatus generally prints while scanning a printhead on a printing medium. The carriage to which the printhead is mounted must achieve higher precision and higher speed. In order to meet these demands, conventional printing apparatuses employ a so-called servo-mechanism which drives a carriage while detecting displacement information of the carriage by a linear encoder.
FIG. 8 is a block diagram showing the schematic configuration of the servo-mechanism of a carriage in a conventional printing apparatus.
As shown in FIG. 8, a carriage 1 to which a printhead is mounted is driven by a belt 3. The belt 3 is fixed to the carriage 1 via a belt holder 4. The belt 3 is suspended between a pulley 6 and an idle pulley 7 without any slackness. The pulley 6 is coupled to a carriage motor 5 serving as a drive source. A torque generated by the carriage motor 5 is converted into a thrust which drives the carriage 1 in the scanning direction via the pulley 6, idle pulley 7, and belt 3.
Displacement information of the carriage 1 is detected by a linear encoder 18. Scanning of the carriage 1 on a printing medium requires displacement information and velocity information of the carriage 1. The velocity information is extracted on the basis of an output signal from the linear encoder 18. A velocity detector 12 generates velocity information on the basis of an output signal from the linear encoder 18. The velocity information generation method is known well. For example, velocity information is generated by measuring the time width of a series of pulses output from the linear encoder 18 or calculating the change amount of the series of pulses per unit time.
The obtained velocity information of the carriage 1 undergoes comparison and subtraction with an output from a velocity instruction value generator 10 by a comparator 30. The result is supplied to a velocity compensator 11, and properly compensated into a control signal for driving the carriage motor 5 via a power amplifier 16.
The servo-mechanism of the conventional carriage forms a feedback loop pertaining to velocity information of the carriage 1.
FIG. 9 is a block diagram for explaining the operation of the conventional servo-mechanism in detail.
The operation of the servo-mechanism will be further explained with reference to FIG. 9. In FIG. 9, the operation of a power-to-thrust conversion mechanism 15 is to apply a thrust to the carriage 1 in accordance with an output from the velocity compensator 11. The power-to-thrust conversion mechanism 15 is comprised of the power amplifier 16, carriage motor 5, pulley 6, idle pulley 7, belt 3, and the like. The carriage 1 is mechanically one rigid body, and an acceleration corresponding to the thrust appears in the carriage 1. The acceleration is proportional to the thrust and inversely proportional to the mass of the carriage 1. A velocity supplied to the velocity compensator 11 is expressed as the first order integration of the acceleration. In general, the performance of the servo-mechanism is evaluated by traceability to a target value and external disturbance suppression. The servo-mechanism of the conventional carriage is designed to achieve these two performance capabilities by feeding back velocity information.
However, it is known well that only velocity feedback cannot provide satisfactory external disturbance suppression.
Influential external disturbance factors are as follows.
First, there are a characteristic drift caused by the temperature rises of the power amplifier 16 and carriage motor 5, and the influence of the counter electromotive voltage of the carriage motor 5. Also, variations in mechanical load torque and the torque ripple of the carriage motor 5 act as an external disturbance force on the servo-mechanism. The conventional system which feeds back velocity information does not have sufficient external disturbance suppression, and variations in the velocity of the carriage 1 upon scanning the carriage are unavoidable.
In order to improve external disturbance suppression of the servo-mechanism, for example, a current feedback power amplifier has conventionally been used. According to this method, the current of the carriage motor 5 is managed by feedback control. However, the thrust which acts on the carriage 1 is not directly managed, and the influence of external disturbance factors cannot be sufficiently eliminated.
External disturbance suppression is also improved by forming multiple feedback loops for the velocity and acceleration of the carriage (see, e.g., Japanese Patent Publication (JPB2) No. 2,784,002).
Japanese Patent Publication No. 2,784,002 discloses an acceleration-controlled servo system.
FIG. 10 is a block diagram showing a state in which Japanese Patent Publication No. 2,784,002 is applied to the servo-mechanism of a carriage in a printing apparatus.
In FIG. 10, an acceleration compensator 13 is arranged on the output side of the velocity compensator 11, and an acceleration feedback loop is formed within a velocity feedback loop. The acceleration of the carriage 1 is integrated by an integrator circuit 22 to obtain the velocity. This method is very effective in principle, but Japanese Patent Publication No. 2,784,002 does not explicitly specify any practical means about how to detect the carriage acceleration at high precision. Japanese Patent Publication No. 2,784,002 assumes that a motor and a mechanism to be controlled (carriage in a printing apparatus) are rigidly coupled. A tachometer is attached to the motor, and a velocity signal output from the tachometer is differentiated to obtain an acceleration signal.
However, the acceleration signal disclosed in Japanese Patent Publication No. 2,784,002 relates to the rotation of the motor, and not to the carriage. This can be ignored if the motor and carriage are rigidly coupled. However, in the printing apparatus, a belt which is a flexible member is used as a force transmission mechanism, and dynamics exists between rotational motion of the motor and translational motion of the carriage. That is, the rotation angular acceleration of the motor cannot substitute for the acceleration of the carriage. Even if the method disclosed in Japanese Patent Publication No. 2,784,002 described above is applied to a printing apparatus, i.e., the tachometer is attached to the motor in the printing apparatus, no intended servo-mechanism can be implemented.
The carriage acceleration can be directly detected by attaching an acceleration sensor to the carriage. However, the acceleration sensor is generally very expensive, and implementation of the acceleration sensor in the printing apparatus is not practical in terms of the cost.
Also, in a case where the linear encoder shown in FIG. 8 is employed, the encoder must be a high-resolution type to obtain acceleration information with sufficient precision. This results in increasing the cost of the apparatus.
As described above, since the servo-mechanism of a carriage employed in a conventional printing apparatus performs velocity information feedback as a basic control system, the conventional printing apparatus cannot attain satisfactory external disturbance suppression. Variations in carriage velocity cannot be suppressed upon scanning the carriage, resulting in printing unevenness in the carriage scanning direction.